The major objective of this proposal is to investigate the etiology responsible for the ongoing chronic fatigue and muscle weakness in veterans with the Persian Gulf illness. For this purpose we will evaluate skeletal muscle function of Persian Gulf Veterans with severe chronic fatigue and Persian Gulf veterans who were deployed but who have no medical problems. We hypothesize muscle function is impaired in Persian Gulf veterans with a symptom profile consistent with CFS. In addition, we hypothesize that the severity of chronic fatigue in this population is related to the degree of muscle dysfunctioning. To test these hypotheses a battery of tests will be performed on Persian Gulf veterans with severe chronic fatigue and on control veterans with no medical complaints. These measurements will include 31P-Magnetic Resonance spectroscopy (MRS), Magnetic Electrodiagnostic evaluation of motor unit recruitment, Muscle enzyme assays, Isokinetic and Isometric testing, and Functional Status Questionnaire. Complementary to the functional tests, the subjects will be screened for AMP deaminase (AMPD) deficiency.